The present invention relates to a method of extracting automatic-focusing information utilized for automatically focusing a focusing lens system in a photographic camera or a video camera.
There are well known in the art automatic focusing methods for extracting automatic-focusing information from a video signal generated by an image sensor or an imager and for adjusting a focusing lens system based on the extracted automatic-focusing information for automatically focusing the focusing lens system.
The automatic-focusing information is normally extracted from one line of an output signal produced by the image sensor. The output signal may not necessarily be of a sufficient amplitude at all times since the output signal tends to vary dependent on the conditions of an object being photographed or imaged. If an output signal of a small amplitude from the imager were converted to a digital signal, the digital signal would contain certain indefinite elements not effective to perfect arithmetic operations required for automatically focusing the focusing lens system because the information contained in the digital signal would be insufficient, resulting in a malfunction of the focusing lens system.
One conventional solution has been to employ more lines over the output signal for extracting automatic-focusing information and then to add the respective pieces of information from the lines in an analog manner. In this prior process, the number of lines for extracting automatic-focusing information is increased to accumulate focusing signals when the contrast of the object is lowered to reduce the amplitude of the output signal from the imager. Therefore, the area in which to extract the automatic-focusing information is widened, and the information is apt to be affected by a larger-size object present in the area. This is problematic in that the focusing lens system finds difficulty in focusing on a smaller object in the area. Stated otherwise, the larger object in the area produces more automatic-focusing information than the smaller object in the area does. Consequently, when there are larger and smaller objects close to each other in one area, the focusing lens system is likely to be focused on the larger object.
Performing the foregoing process requires a substantial area of the imager to be used for processing output signals for extracting automatic-focusing information. This automatic-focusing process is not suitable for extracting automatic-focusing information for relatively small objects, and tends to be influenced by an object which takes up a larger area on the imaging surface.
The foregoing conventional process cannot be relied on in the case where the imager is a one-dimensional image sensor.